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Chapter 3 - Amount of substance I - Coggle Diagram
Chapter 3 - Amount of substance I
Amount of substance and the mole
Amount of substance (n) is used to count the number of particles in a substance
Measured in a unit called the mole
One mole is the amount of substance that contains 6.02x10^23 particles
The Avogadro constant is the number of particle in each mole of carbon-12
To find the mass of one mole of atoms of any element, its just the relative atomic mass in grams
Molar mass
Way of linking moles with mass
Gives the mass in grams of each mole of the substance
Molar mass in the mass per mole of a substance
Units are g mol^-1
Formulae
Molecular formulae
Number of atoms of each element in a mole
Molecule - two or more atoms held together by covalent bonds
Empirical formulae
Simplest whole number ratio of atoms of each element in a compound
Important for substances that do not exist as molecules
It would be impossible to base a formula on the actual number of atoms or ions - they would go into the billions
Empirical formula is the ratio of atoms or ions in the structure and will always be the same
Relative molecular mass
The average mass of one molecule of an element or compound compared to 1/12th the mass of an atom of carbon-12
To calculate relative molecular mass add together the relative atomic masses of the elements that make up the molecule
Relative formula mass
Calculated by adding together the relative atomic masses of the elements in the empirical formula
The weighted mean mass of a formula unit compared to 1/12th the mass of an atom of carbon
Hydrated salts
Many coloured salts are hydrated, water molecules are part of their crystalline structure
This water is known as water of crystallisation
When blue crystals of hydrated copper(II) sulphate are heated, bond holding the water within the crystal are broken and water is driven off, leaving anhydrous copper(II) sulphate
Without water, the crystalline structure is lost and a white powder remains
It is very hard to remove all the water
Accuray
Assumptions 1
If the hydrated and anhydrous forms have different colours, you can be fairly sure all the water has been removed
However you only see the surface of the crystals, some water could still be inside
A good solution is to heat to a constant mass, suggesting all the water has been removed
All the water has been lost
Assumption 2
No further decomposition
Many sals decompose further when heated
Copper(II) sulfate decomposes to form black copper(II) oxide
This can be very difficult to judge if there is no colour change
Anhydrous - doesn't contain water of crystallisation
Moles and volumes
Moles and solutions
To work out the amount of a measured volume, you need to know the concentration of the solute (dissolved compound)
The concentration is the amount of solute dissolved in each 1dm^3 of solution
To concert between concentration and moles, multiply the concentration by the volume in dm^3
Standard solutions
Solution of known concentration
Prepared by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume
Moles and gas volumes
At the same temperature and pressure, equal volumes of different gases contain the same number of moles
Molar gas volume
volume per mole of has molecules at a stated temperature and pressure
Volume of gas depends on the pressure and temperature, but most experiments are carried out at rtp
RTP is about 20 degrees and 1atm
At RTP, 1 mole of gas has a volume of 24dm^3
Converting beween moles and gas volumes
Amount = volume / 24000cm3 or 24dm3